Building a car-parking platform

Comments

I've got a construction project that seems to be unusual enough that (a) I've yet to see discussion of the relevant issue on this board or on elsewhere on the net, and (b) I'm not even sure if this question is best posed in the "Decks" or "Concrete" sections, or somewhere else.
My rowhouse on a narrow lot (12') has a small backyard (35'deep) that backs up to an alley. However, the rear of the yard is approximately 40" below the level of the alley, and there is a concrete block retaining wall. The yard itself slopes away from the alley and toward the house (where all the water collected in the backyard drains into a small drain immediately outside the rear entrance to the house -- and can get cloggecd during heavy rains).
I would like to build a "deck" or other type of platform that would span the 12' width of the yard, be even with the alley level at the rear of the yard, and be approximately 19' long. The platform would serve several functions: It would slope gently (2%) toward the alley, thus improving the drainage situation. It would allow for storage of tools, boats, and bikes in the "crawlspace" underneath it. And (importantly) it would provide a parking space accessible from the alley (but could be used as a traditional deck or patio when not used for parking). So it needs to be able to support the weight of a car, and the surface must be water-impermeable.
The plan I have in mind is to build a platform using concrete-block walls on three sides for support (one wall would abut the existing retaining wall at the rear of the yard), using 3 steel I-beams to span the width between the side-walls, putting steel plates over the beams, and then pouring a thin layer of concrete on top of the steel plates.
Here are some figures I have in mind:
Total anticipated live load would be 5000 lbs. This includes the weight of a typical car (not SUV) parking here (2500 lbs.), plus occupants, and assorted junk. Or, if used for a deck, this would allow 25 people to squeeze on. Size would be 228 sf, so anticipated LL/sf would be under 20 psf. With the design described above, assuming the use of lightweight concrete, 4" thick (@ 50 psf), the dead load would be in the neighborhood of 65 psf. The total (live + dead) load would be almost twice what I understand the "normal" total load for a traditional wooden deck to be (i.e. 40 psf LL + 10 psf DL).
I am seeking comments on this idea. For example, would 3 steel (6"?) i-beams (spanning the width at the front, back, and middle of the platform) provide sufficient support for the steel/concrete decking, or do I need to run a series of joists on top of these cross beams as well? Perhaps would it be better to use beams placed strategically under the areas where the car's wheels will rest? Is it overkill to use a surface this thick and heavy? Suggestions for alternatives that might be lighter/cheaper/less laborious/etc. would be welcome as well.

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